In electrolytic cells for the production of hydrogen and oxygen, such as those of the bipolar and unipolar type, an aqueous caustic solution is electrolyzed to produce oxygen at the anode and hydrogen at the cathode with the overall reaction being the decomposition of water to yield hydrogen and oxygen. The products of the electrolysis are maintained separate by use of a membrane/separator. Use of amorphous metals/metallic glasses and nanocrystalline materials, as electrocatalysts for the hydrogen and oxygen evolution reaction are known. The terms "amorphous metal" or "metallic glass" are well understood in the art and define a material which contains no long range structural order but can contain short range structure and chemical ordering. Henceforth, in this specification and claims both terms will be used as being synonymous and are interchangeable. The term "nanocrystalline" refers to a material that possesses a crystallite grain size of the order of a few nanometers; i.e. the crystalline components have a grain size of less than about 10 nanometers. Further, the term "metallic glass" embraces such nanocrystalline materials in this specification and claims.
In an electrolysis application, not all of the voltage that is passed through the cell during electrolysis is utilized in the production of hydrogen and oxygen. This loss of efficiency of the cell is often referred to as the cell overpotential required to allow the reaction to proceed at the desired rate and is in excess of the reversible thermodynamic decomposition voltage. This cell overpotential can arise from: (i) reactions occurring at either the cathode or the anode, (ii) a potential drop because of the solution ohmic drop between the two electrodes, or (iii) a potential drop due to the presence of a membrane/separator material placed between the anode and cathode. The latter two efficiencies are fixed by the nature of the cell design while (i) is directly a result of the activity of the electrode material employed in the cell including any activation or pre-treatment steps. Performance of an electrode is then directly related to the overpotential observed at both the anode and cathode through measurement of the Tafel slope and the exchange current density (hereinafter explained).
Superior electrode performance for the electrolysis of water may be achieved by the use of addition of metal salts to the electrolyte as "homogeneous" catalysts that function only in the liquid phase. These "homogeneous" catalysts suffer from the difficulty of having to add these additions to an operating cell to be functional, along with the toxicity of the metal salts in powder form and the disposal of electrolyte containing these additions. A desirable alternative would then be a base alloy comprised of Ni, and one or more of these metallic salt constituents which would still provide the same operating characteristics of a low voltage, high current cell behaviour corresponding to the evolution of hydrogen or oxygen while being electrochemically stable in the alkaline solution.
U.S. Pat. No. 5,429,725, issued Jul. 04, 1995 to Thorpe, S. J. and Kirk, D. W. describes the improved electrocatalytic behaviour of alloys made by combinations of the two elements Mo and Co in a Ni-base metallic glass.
However, this is still a need for higher exchange current densities combined with lower Tafel slopes in the (Cr, V)-containing alloys compared with the Mo-containing alloys and, accordingly, a need for enhanced operating efficiency of electrocatalyst material for alkaline water electrolysis